1. Field of the Invention
The present invention relates to an apparatus and method for measuring velocity. More particularly, the present invention relates to a velocity correlator utilizing side-looking sonar (SLS) which is useful in underwater vehicles for measuring speed relative to the bottom.
2. Discussion of the Related Art
One conventional technique used in measuring the speed at which an underwater vehicle is travelling relative to the bottom is the Doppler sonar method. To measure the forward velocity of the underwater vehicle using the Doppler sonar technique, two sonar projectors are used which are positioned such that one of the projectors projects a sonar beam in the forward direction and the other projects a sonar beam in the aftward direction of the vessel. When the vessel is moving forward, the received echo of the fore directed beam will exhibit an upward shift in Doppler frequency and the received echo of the aft directed beam will exhibit a downward shift in Doppler frequency. The faster the vessel moves in one direction, the greater the shifts in Doppler frequency. Thus, the velocity of the vessel may be determined by measuring the shifts in Doppler frequency.
The Doppler sonar technique is susceptible to error, however, since the wide beams have spreads of Doppler shifts and the bottom scattering characteristics are not always uniform. Also, bottom slopes cause errors.
In an effort to provide for greater range to the bottom, the correlation velocity log sonar was developed. The correlation velocity log sonar utilizes a single sonar projector which projects a sonar beam vertically downward. A pair of receivers, which are directed downward and spaced apart from one another along the longitudinal axis of the vessel, sense the echo from the projected sonar beam and provide identical output waveforms. When the vessel is moving forward, the aft receiver will sense the echo after the forward receiver causing the output wave form of the aft receiver to be delayed from that of the forward receiver. This delay is equal to the distance between the two receivers divided by twice the velocity of the vessel. Thus, the velocity of the vessel may be determined by measuring the delay between the output signals of the fore and aft positioned receivers having a fixed distance therebetween or, alternatively, by fixing the measurable delay time and altering the spacing between the fore and aft receivers.
Because it is necessary that the delay in which the aft receiver receives the same return echo as the fore receiver is determined precisely, and because the return echo received by the receivers varies when a continuous wave is transmitted by the projector, it is necessary to correlate the output waveforms of the fore and aft receivers and to determine what delay or spacing is required to maximize the correlation.
When the fore and aft positioned receivers have a single fixed distance therebetween, it is necessary to vary a delay introduced into the output waveform of the fore receiver until a correlation of the delayed output waveform of the fore receiver and the output waveform of the aft receiver is maximized. The delay introduced into the output waveform of the fore receiver and the distance between the fore and aft receivers can then be used to calculate the velocity of the vessel.
As an alternative method, the spacing between the fore and aft receivers can effectively be altered by replacing the single aft receiver with an array of receivers having identifiable fixed spacings between each receiver of the array and the forward receiver. Then, by fixing the delay introduced into the output waveform of the fore receiver, the output waveform of the fore receiver is correlated with the output waveform of each of the aft receivers. The correlations are compared to determine which of the aft receivers has output a waveform that has the maximum correlation with the output waveform of the fore receiver. The distance between the fore receiver and the aft receiver having maximum correlation is then used to determine the velocity of the vessel. Further, to more accurately determine the proper spacing, the spacings between the fore receiver and the aft receivers having the two highest levels of correlation may be used to interpolate the proper spacing.
Doppler sonar and correlation velocity log sonar have, at best, accuracies of approximately 0.05 knot.